1. Field
This invention relates to improvements in high frequency semiconductor devices and particularly to improvements in the reduction of skin effect loss in millimeter wave semiconductor diodes.
2. Prior Art
The flow of an alternating current in a conductor produces a field that tends to constrain the current to a region near the surface or "skin" of the conductor. This characteristic, generally known as the skin effect, induces a loss that is present to some extent to all frequencies, but is particularly noticeable at microwave and higher frequencies. At frequencies where the skin effect can be neglected, the current is generally evenly distributed throughout the cross section of a conductor. At frequencies where the skin effect cannot be neglected, the current is forced to flow through the relatively narrow cross section of the "skin" and along surface paths that often increase the total path length and the loss.
The term N doped material, as used herein, refers to extrinsic semiconductor material in which the electron carrier concentration exceeds the mobile hole concentration. N+ material refers to N doped material in which the electron carrier concentration is high. P material refers to material in which the hole carrier concentration exceeds the electron concentration and P+ refers to material in which the mobile hole carrier concentration is high. The carrier concentration in P or N material ranges from about 10.sup.16 to 5.times.10.sup.17 per cubic centimeter, whereas the carrier concentration in P+ or N+ material ranges from about 1.times.10.sup.18 to 1.times.10.sup.19 per cubic centimeter. Materials with high carrier concentrations such as N+ and P+ materials have relatively low resistivity and conversely materials with low carrier concentrations such as N and P materials have relatively high resistivity. The exact relationship between the resistivity and the carrier concentration for various material may be found in a number of publications including S.M. Sze, "Physics of Semiconductors," Wiley Interscience, New York, 1969.
FIG. 1 is a cross sectional view of a prior art silicon planar diode showing the current paths through the diode at both low and millimeter wave frequencies. In this device, a semiconductor junction 3 is formed at the interface of a P doped layer 2 and an N doped layer 4. The N doped layer contacts a body of N+ material 7 at a second interface 6. A first electrode 1 makes contact with the upper surface of the P layer. A second electrode 8 makes contact with the N+ body on the lower surfaces of the device. A silicon dioxide passivation layer 15 protects a portion of the upper surface of the N layer about the junction.
The current flow through the diode from the junction to the second electrode at low frequencies is represented by a current path I.sub.o, identified by drawing numeral 14. The current flow at millimeter frequencies is represented by current paths I.sub.1 through I.sub.5, identified by drawing numeral 9 through 13 respectively.
The term "skin depth" refers to the depth in a material at which the field strength is 37 percent of that at the surface. The skin depth in FIG. 1 is represented by dimension line 5. The current path I.sub.1 shows the depth of penetration of the initial downward flow of the current immediately beneath the junction to be approximately one skin depth.
After the initial downward flow of the current through the N layer into the N+ material the current direction changes towards the sides of the diode, as shown by the I.sub.2 current path. After leaving the junction area, the current follows a path along the surface of the diode. I.sub.3, I.sub.4 and I.sub.5 represent the current paths along the diode upper surface, sides and lower surface, respectively. Symbols R.sub.1 through R.sub.5 represent the values of resistance in paths I.sub.1 through I.sub.5, respectively. The diode resistance from the junction to the second contacting layer is the sum of R.sub.1 through R.sub.5.
At low frequencies, the skin effect is negligible and the current flow is generally in a direct path from the junction through the N layer and N+ body to the second electrode. Although the current at these frequencies emanates from the entire undersurface of the junction, it is represented in FIG. 1, for the sake of clarity, by a single current path line 14.
The path length through the N and N+ materials at low frequencies is short, as can be seen from the I.sub.o path in FIG. 1. However, at millimeter wave frequencies, this short path is replaced by the much longer I.sub.1 through I.sub.5 paths. This longer path length and the relatively small cross sectional area of the skin combine to produce a high skin effect loss in planar devices.
The device loss can be further increased by the fringing capacitance. This is the capacitance between the first electrode and the upper semiconductor layer of the device, such as the N layer in FIG. 1. It produces a factor that multiplies the diode series resistance.
For low loss operation at millimeter wave frequencies, the fringing capacitance must be kept to a minimum. Unfortunately, the planar diode of FIG. 1 does not exhibit a low fringing capacitance. In this device, the first electrode extends beyond the P layer over the N layer, with only a thin layer of silicon dioxide separating the two. This structure is in effect a parallel plate capacitor that produces a relatively high value of fringing capacitance and a correspondingly high value of equivalent series resistance.
A method of reducing the skin effect loss in millimeter wave diodes is described by Anderson et al., in U.S. Pat. No. 3,387,189. A cross sectional view of the Anderson device is shown in FIG. 2. The Anderson device is similar to the device shown in FIG. 1 in that it is a planar diode, but there are several important differences. The silicon dioxide passivation on the Anderson device extends only a short distance from the first electrode. The remaining areas on the upper surface and sides of the device are covered by a metal film conductor 16. Although the metal film conductor is shown in FIG. 2 as comprising a single material, this film may, in a practical device, include a contacting layer of one material and an electrode layer of a different material. The purpose and composition of these layers is explained in more detail in the description of the devices shown in FIGS. 3 and 4. The P layer in the Anderson device is diffused into the N layer at least one skin depth, as shown by dimension line 5 in FIG. 2. This diffusion depth represents a significant increase in the thickness of the P layer over the device shown in FIG. 1.
In the operation of the Anderson device at millimeter wave frequencies, the initial flow of current from the junction, represented by paths I.sub.1 and I.sub.2, is similar to that shown in FIG. 1. However, the path represented by I.sub.3 in FIG. 1 is changed in FIG. 2. The equivalent current flow in the Anderson device starts at the edge of the junction and then flows upward towards the metal film conductor 16. This new current path is designated I.sub.3 ' and is identified in FIG. 2 by drawing numeral 12. The resistance associated with the I.sub.3 ' path is represented by the symbol R.sub.3 '. This resistance includes the contact resistance at the interface of the semiconductor material and the metal film 16. The total resistance from the junction to the metal film conductor is the sum of R.sub.1, R.sub.2 and R.sub.3 '.
The principal purpose of the Anderson design is to reduce the skin effect loss. The metal film conductor on the diode surface does provide a lower resistance path for the current than is provided by the semiconductor material, but there are a number of factors in the Anderson design that tend to negate the advantage of the metal film conductor. These factors include high fringing capacitance, high contact resistance at the interface of the metal film and the N layer, and an increase in portions of the current paths through the P and N layers.
There is no improvement in the Anderson device over the planar device shown in FIG. 1 in reducing the fringing capacitance. As a result, the high fringing capacitance and high loss associated with this capacitance in planar devices remain present in the Anderson device. The metal film conductor makes contact with the N layer rather than with the N+ material. The resulting contact resistance is relatively high. The increased diffusion depth of the P layer requires the current to flow through a relative long path in the high resistance P and N layers before it reaches the metal film conductor, thereby increasing the diode series resistance.
In addition to these factors, the silicon planar structure of the Anderson device exhibits other undesirable characteristics including susceptible to accelerated degradation, premature breakdown and unsatisfactory performance in detector and mixer application. The silicon dioxide passivation of the planar structure is susceptible to sodium ion migration, an important factor in accelerating device degradation. High fields produced at the periphery of the junction in planar devices result in premature breakdown and early device failure. The capacitance formed by the N layer and the extension of the junction electrode over the silicon dioxide layer, as shown in FIGS. 1 and 2, result in charge accumulation at the interface of the silicon dioxide coating and the N layer. This charge accumulation modifies the space charge distribution, tending to make device performance unpredictable. Finally, storage capacitance in the P layer produced by minority carrier injection prevents successful use of the Anderson device in mixer and detector application.
Many of the semiconductor materials and fabrication techniques currently available were not available, in a practical sense, at the time the Anderson device was developed. The Anderson design is based on the materials and techniques available at the time it was developed and therefore this device cannot be considered as optimum by present day standards. For example, the silicon material of the Anderson device is not optimum for high frequency applications because of its relatively high resistivity. Gallium arsenide is preferred for this type of application because of its lower resistivity. However, until recently gallium arsenide was not widely used because of problems encountered in providing a satisfactory passivation coating and an economically practical gold diffusion shield. A method of overcoming these problems is now available and is described in my U.S. Pat. No. 3,923,975.